1. Field of the Invention
The present invention relates to downhole apparatus and particularly, but not exclusively, to multi-cycle circulating subs used during downhole drilling operations.
2. The Prior Art
It is often necessary in downhole drilling operations to bleed the flow of wellbore fluid down the drill string into the wellbore annulus. For example, this may be necessary where the desired fluid flow rate to drive a drilling tool is insufficient to carry all the drilled material up the annulus to the surface. In these circumstances, a circulating sub may be used to allow the flow rate required to remove the drilled material to be pumped into the annulus whilst maintaining the lower flow rate required at the drilling tool.
It is known to provide a circulating sub with an axially movable piston for opening and closing vent apertures. The vent apertures are provided in a body of the sub and allow wellbore fluid pumped downhole through a central bore of the sub to pass into the surrounding wellbore annulus. Opening and closing of the vent apertures by means of the piston is controlled by a pin and groove arrangement. The pin is located in one of the piston and body and is received within the groove provided in the other of the piston and body. The profile of the groove is such that axial movement of the piston results in rotation of the piston within the body. Furthermore, the extent of axial piston movement is limited by the groove profile. Thus, the piston may be moved axially downhole by means of a predetermined fluid flow rate and returned uphole by means of a biasing spring so as to cycle the piston into a position wherein the control groove permits subsequent movement of the piston from a vent aperture closed position to a vent aperture open position.
A problem associated with the aforementioned prior art means for controlling the piston results from the helical compression spring generally used to bias the piston uphole. As the piston is pressed downhole by a fluid flow so as to compress the spring, there is a tendency for the spring to grip the piston and apply a rotational force thereto. This rotational force can often be in opposition to the control groove and pin. For example, in a movement of a piston from a vent aperture closed position to a vent aperture open position, a control groove will typically have a profile which is intended to allow for axial piston movement without any rotation of the piston relative to the body. In these circumstances, it is known for the rotational force applied by the spring to undesirably shear the control pin within the control groove.